(1) Field of the Invention
This invention relates to an LED driving circuit and an LED driving controller.
(2) Description of the Prior Art
FIG. 1 is a circuit diagram of a typical LED driving circuit. The LED driving circuit includes an inductance L, a transistor switch M1, a flywheel unit M2, an LED module 20, a current detecting resistance Ri and a controller 10. The controller 10 controls the transistor switch M1 to be turned on/off by means of constant off-time control, so as to drive the LED module 20 lighting. The description of the detailed circuit operation is as following.
The controller 10 includes a comparator 2, a SR flip-flop 6 and a constant off-time unit 4. A non-inverting end of the comparator 2 receives a reference signal Vr and an inverting end thereof is coupled to the current detecting resistance Ri to receive a current detecting signal IFB. At the beginning of each cycle, the transistor switch M1 is turned on and so an increasing current supplied by an input power source Vin flows through the inductance L, the LED module 20 to be grounded. When a level of the current detecting signal IFB is higher than that of the reference signal Vr, the comparator 2 generates a high-level signal to a set end S of the SR flip-flop 6. Therefore, the SR flip-flop 6 is triggered to output a low-level signal at an inverting output end Q′ to cut the transistor switch M1 off. Afterward, the current flowing through the LED module 20 freewheels through the flywheel unit M2, and an output end Q of the SR flip-flop 6 generates a high-level signal to the constant off-time unit 4. After a predetermined off-time period, the constant off-time unit 4 generates a pulse signal to a reset end R of the SR flip-flop 6, and so the SR flip-flop 6 generates a high-level signal at the inverting output end Q′ to turn on the transistor switch M1 again for the next cycle.
In theory, the current of the LED module 20 is vibrated between a current peak value and a current valley value. However, the current valley value depends on the predetermined cut-off time period, and varies with a voltage of the input power source Vin. It results that an average value of the current is changed due to the input power source Vin, as same as an illumination and a color-temperature of the LED module 20. Even the actual current peak value of the current the LED module 20 also varies because of time delay in circuit operation and the process error in the inductance value of the inductance L.